1. Field of the Invention
The present invention relates, in general, to the removal of contaminants from flue gas and in particular to a new and useful system and method for removing SO.sub.2 from a heat exchanger in a regenerative scrubber system.
2. Description of the Related Art
There are several systems used for removing contaminants from flue gas. Many of these systems which pertain to the removal of sulfur oxides (SO.sub.2) and other contaminants are disclosed in Power Magazine published in May, 1993. In these systems, slurry-based sorbent is employed in a once-through system in order to remove sulfur oxides, particulates and mercury from flue gases; and the reaction products are then drained for discharge treatment.
There are also a number of known regenerable processes used for flue gas desulfurization. These include sodium based systems such as SOXAL (trademark owned by Allied Corporation, AQUATECH Systems), the Wellman-Lord process, the citrate process, and MgO process. Wet regenerable processes generally have an absorption step in which fresh or regenerated reagent is reacted with SO.sub.2 in order to clean the flue gas. The products of the SO.sub.2 absorption reaction are then sent to a regeneration system which generally involves the addition of heat to produce an SO.sub.2 rich gas that can be further treated to produce sulfur, sulfuric acid or SO.sub.2. The reagent is produced, i.e. regenerated for reuse in the SO.sub.2 absorption process. For a sodium scrubber, the general reactions involved are: ##STR1##
The regeneration for these processes is complicated and consumes a large amount of energy.
Similar systems, known as dual-alkali processes, use one reagent (generally sodium carbonate/sodium sulfite) to absorb SO.sub.2. The product of this reaction (sodium bisulfite) is then reacted with lime or limestone to form calcium sulfite and calcium sulfate in order to regenerate the reagent. These dual-alkali processes produce a sludge that must be landfilled or converted to a useful form. The reactions for the dual alkali process using limestone and lime are found in the following reactions: EQU 2NaHSO.sub.3 (aq)+CaCO.sub.3 .fwdarw.1/2 H.sub.2 O+CaSO.sub.3 .cndot.1/2 H.sub.2 O.dwnarw.+CO.sub.2 .uparw.+Na.sub.2 SO.sub.3 (aq) (4) EQU 2NaHSO.sub.3 (aq)+Ca(OH).sub.2 .fwdarw.Na.sub.2 SO.sub.3 (aq)+3/2 H.sub.2 O+CaSO.sub.3 .cndot.1/2 H.sub.2 O (5)
Non-regenerated flue gas desulfurization (FGD) systems are generally calcium based. In these systems, lime or limestone is reacted with SO.sub.2. The end product is often a sludge similar to that produced by dual-alkali systems.